1. Field of the Invention
The present invention is in the field of calcining aluminum hydroxide to produce aluminum oxide, and includes a drying means for the material to be calcined, a two-stage calcining means, and a cooling means.
2. Description of the Prior Art
Calcination of aluminum hydroxide source materials was heretofore carried out in mainly two different types of calcining apparatus, in rotary tubular kilns or in so-called stationary systems such as cyclones or in fluidized bed calciners. The calcination in a rotary tubular kiln is the best known and most widely used. The rotary tubular kiln permits the production of all grades of aluminum oxide up to special alumina sources containing practically pure .alpha.-content. When calcining in the rotary tubular kiln, however, the ratio between the energy input and the quantity of the product produced is unfavorable particularly with alumina sources containing up to about 80% of the .alpha.-modification due to the poor energy transfer effect since there is no adequately intensive intermixture of hot gas stream and product.
The calcination of aluminum hydroxide material to aluminum oxide having an .alpha.-content below 80% in a so-called fluidized bed reactor yields a considerable energy savings in comparison to the rotary kiln. Thus, about 25 to 30% of the specific fuel consumption can be saved when calcining the aluminum oxide source in a fluidized bed reactor. High .alpha.-containing aluminum oxides having .alpha.-contents over 80%, however, cannot be continuously produced in such calciners. Due to the high temperatures required, phases occur which lead to cakings and sinterings of the alumina sources and can thereby plug the calciner.
Example 3 of German AS No. 1 184 744 shows an installation wherein a three-stage cyclone heat exchanger is provided upstream of a rotary tubular kiln. In accordance with this known system, the specific heat consumption is to be lowered, so that a short duration of the heating of the aluminum hydroxide occurs with the assistance of a burner in the conduit to the cyclone stage, leading to a temperature which triggers the exothermic conversion process into the .alpha.-modification which then proceeds autonomously. The initiation of the conversion process proceeds in a cyclone system. The rotary tubular kiln which follows downstream thereby serves only as a reaction space for the autonomous conversion of the aluminum hydroxide which then occurs. The burner provided in the rotary tubular kiln is used exclusively to compensate for the thermal losses that arise. As indicated in Example 3 of the reference, the maximum obtainable .alpha.-proportion lies at about 70%.